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Schedule of Intangible Assets, Including Impact of Impairment Charges (Detail) - USD ($)
$ in Thousands
 12 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Finite Lived Intangible Assets [Line Items]    
Intangible Assets, Accumulated Amortization $ (72,477) $ (69,269)
Goodwill, Cost 205,211 205,211
Intangible Assets, Impairment (6,841)  
Goodwill, Net 198,370 205,211
Total Goodwill and Intangible Assets, Cost 308,611 308,611
Goodwill and Intangible Assets, Impairment (7,019)  
Goodwill and Intangible Assets, net $ 229,115 $ 239,342
Customer Contracts And Relationships [Member]    
Finite Lived Intangible Assets [Line Items]    
Intangible Assets, Useful Life 15 years 15 years
Intangible Assets, Cost $ 72,260 $ 72,260
Intangible Assets, Accumulated Amortization (61,009) (58,732)
Intangible Assets, Impairment (178)  
Intangible Assets, Net $ 11,073 $ 13,528
Sales Contracts [Member]    
Finite Lived Intangible Assets [Line Items]    
Intangible Assets, Useful Life 4 years 4 years
Intangible Assets, Cost $ 2,500 $ 2,500
Intangible Assets, Accumulated Amortization $ (2,500) $ (2,500)
Permits [Member]    
Finite Lived Intangible Assets [Line Items]    
Intangible Assets, Useful Life 40 years 40 years
Intangible Assets, Cost $ 28,640 $ 28,640
Intangible Assets, Accumulated Amortization (8,968) (8,037)
Intangible Assets, Net $ 19,672 $ 20,603